Electronic devices are rapidly becoming more miniaturized and reduced in weight, while the mounting densities of electronic parts mounted on electronic devices are also increasing. To respond to this trend, a greater variety of properties are being required for electronic parts and the materials composing them.
For example, thinner printed circuit boards are in demand as electronic parts suitable for the smaller and thinner sizes of electronic devices. The demand for thinner sizes can only be met by reducing the total thickness, in the case of a multilayer rigid circuit board, for example. This can be achieved by reducing the thickness of the prepreg that forms the board section.
However, since the prepreg is usually obtained by impregnating a glass cloth with a thermosetting resin such as an epoxy resin, there have been technical limits to thickness reduction. Furthermore, reducing the thickness of a prepreg tends to lower the electrical characteristics such as insulation resistance and migration resistance. In addition, attempting to produce a thin prepreg may result in inconveniences such as uneven thickness.
Printed circuit boards that are increasing in demand include flexible multilayer wiring boards (FPC multilayer wiring boards) and rigid-flex printed circuit boards, which are characterized by being suitable for high-speed signals and allowing a high degree of design freedom in electronic devices. Flexible multilayer wiring boards, in particular, wherein the insulating layer composing the substrate is formed as a film, allow the total thickness to be reduced relatively easily.
Patent document 1 indicated below discloses a multilayer circuit board obtained by laminating an outer layer material made of a flexible base on an inner layer material made of a rigid base. However, while it is possible to reduce the total thickness with such multilayer circuit boards, the lack of flexible sections makes it impossible to obtain an adequate degree of design freedom.
As a multilayer circuit board with an increased degree of design freedom, Patent document 2 indicated below describes a flexible multilayer wiring board comprising a cable section composed of an inner layer board comprising a plurality of conductive layers formed on a flexible insulating layer, and an outer layer wiring section obtained by laminating an outer layer board on the surface of an inner layer board.
[Patent document 1] Japanese Unexamined Patent Publication HEI No. 5-041580
[Patent document 2] Japanese Unexamined Patent Publication No. 2006-196762